1. Field of the Invention
The present invention relates to a method of producing an arc tube which is used as a light source of a vehicle headlamp or the like, and more particularly to a method of forming a light emitting tube portion in an intermediate portion of a cylindrical glass tube blank.
2. Description of the Related Art
Since an arc tube can perform bright irradiation, it has recently been used as a light source for a vehicle headlamp or the like.
FIG. 5 shows a typical arc tube as used in, for example, a vehicle headlamp. The arc tube includes an arc tube body 4 made of quartz glass, and a pair of electrode assemblies 6A and 6B. In the arc tube body 4, pinch seals 4b1 and 4b2 are formed on opposite sides of a light emitting tube portion 4a having a substantially elliptical spherical shape, wherein the light emitting tube portion 4a forms a discharge space 2. The electrode assemblies 6A, 6B are pinch-sealed to the pinch seals 4b1 and 4b2 so that their tip ends are protruded into the discharge space 2.
In a process of producing such an arc tube, first, the light emitting tube portion 4a is formed in an intermediate portion of a cylindrical glass tube blank. The formation of the light emitting tube portion 4a is conducted by the steps shown in FIG. 6, and is described below.
As shown in FIGS. 6(a) and (b), a cylindrical glass tube blank G is rotated about its axis Ax while end portions of the cylindrical glass tube blank G are gripped by chucks 12A and 12B. As the cylindrical glass tube blank G is rotated, an intermediate portion A thereof is heated by burners 14 so as to be softened. As the cylindrical glass tube blank G is rotated, and heated, the chucks 12A and 12B are moved a predetermined distance toward each other, thereby gathering softened glass in the intermediate portion A so as to form an intermediate portion B. Thereafter, as shown in FIG. 6(c), a split mold 16 is pressed against opposite sides of the intermediate portion B of the cylindrical glass tube blank G. Under this state, blow molding is conducted by blowing a gas into the cylindrical glass tube blank G, thereby forming the intermediate portion B into a substantially elliptical spherical shape.
Conventionally, as shown in FIG. 7, a chuck moving mechanism is used to cause the chucks 12A and 12B to approach one another in the process of forming a light emitting tube portion.
As illustrated, the chuck moving mechanism 120 includes a chuck supporting mechanism 122 and a chuck pressing mechanism 124.
In the chuck supporting mechanism 122, the chucks 12A and 12B are supported by a guide rod 126 so as to be slidable in the direction of the cylindrical glass tube blank""s axis Ax, and the chucks 12A and 12B can be moved in opposite directions by an interlocking mechanism which is not shown.
On the other hand, the chuck pressing mechanism 124 includes a stepping motor 128, a screw feeding mechanism 130, and a pressing block 132. The pressing block 132 is fixed to a movable portion 130a of the screw feeding mechanism 130, and is driven by the stepping motor 128 to be moved in the direction of the axis Ax.
A roller 136 is attached via a bracket 134 to one of the chucks 12A and 12B, for example chuck 12A. The pressing block 132 abuts against the roller 136 to press the roller 136. This pressing operation causes the chuck 12A to be moved toward the other chuck 12B along the axis Ax. At this time, the chuck 12B also is moved, by the interlocking mechanism, toward the chuck 12A in an interlocked manner with the movement of chuck 12A.
In the process of forming a light emitting tube portion, the shape of the intermediate portion B is a factor which largely determines the thickness and the inner face shape of the light emitting tube portion 4axe2x80x94i.e., the size and the shape of the discharge space 2xe2x80x94that is to be formed in the subsequent blow molding. In order to accurately form the light emitting tube portion 4a to enhance the optical quality of the arc tube, therefore, it is very important for the chuck moving mechanism 120 to correctly control the approach movement, including distance, of the chucks 12A and 12B.
In the conventional process of forming a light emitting tube portion, the approaching movement of the chucks 12A and 12B in the gathering operation is controlled in accordance with the number of pulses which are sent from a controller to the stepping motor 128 in the chuck pressing mechanism 124. Consequently, a problem arises in that the approaching movement of the chucks 12A and 12B is deviated from that desired by, for example, variation in the positional relationship between the pressing block 132 of the chuck pressing mechanism 124 and the roller 136 of the chuck 12A, deformation of the bracket 134, deformation of the roller 136, or backlash of the screw feeding mechanism 130, and the like.
To account for variation in the approaching movement, a method may be employed in which the position of the pressing block 132 is detected during the gathering operation and a result of the detection is fed back to the controller for the stepping motor 128. Even when this method is employed, however, it is impossible to eliminate error caused by variation in the positional relationship between the pressing block 132 and the roller 136, or deformation of the bracket 134, deformation of the roller 136, and the like.
The present invention has been conceived in view of the above-described circumstances. It is an object of the invention to provide a method of producing an arc tube in which, in a process of forming a light emitting tube portion in an intermediate portion of a cylindrical glass tube blank, the light emitting tube portion can be accurately formed.
In the present invention, a method of controlling the approach movement of a chuck in a gathering operation is improved so as to attain the above and other objects and advantages.
The method of the present invention is a method of producing an arc tube in which, in a process of producing an arc tube including a light emitting tube portion having a substantially elliptical spherical shape, the light emitting tube portion is formed in an intermediate portion of a cylindrical glass tube blank, and the method includes:
rotating the cylindrical glass tube blank while gripping end portions of the cylindrical glass tube blank with chucks;
heating a vicinity of the intermediate portion of the cylindrical glass tube blank to soften the intermediate portion;
moving the chucks, by a chuck moving mechanism, a predetermined distance toward each other, thereby gathering softened glass in the intermediate portion; and
expanding the intermediate portion by blow molding into a substantially elliptical spherical shape,
wherein the step of moving the chucks further includes detecting a position of at least one of the chucks by using a position detector, and the method further includes feeding a result of the detection back to the chuck moving mechanism to control an approach distance of the chucks.
The specific configuration of the above-mentioned xe2x80x9cposition detectorxe2x80x9d is not particularly restricted as long as it can detect the position of at least one of the chucks. In order to enhance the detection accuracy, however, it is preferable to use a non-contact position detector such as a laser position detector, an electrostatic capacitance type position detector, an image sensor, or a linear encoder.
As described above, according to the invention, in the process of forming a light emitting tube portion in an intermediate portion of a cylindrical glass tube blank, the pair of chucks gripping the end portions of the cylindrical glass tube blank are movedxe2x80x94by the chuck moving mechanismxe2x80x94so as to approach each other, thereby performing the gathering operation. In this case, the position of at least one of the chucks is detected by using the position detector, and a result of the detection is fed back to the chuck moving mechanism, so that the approach movement of the chucks is controlled. Therefore, the method can attain the following effects.
The position detector detects the position of the chuck itself. Therefore, even when the positional relationship between the chuck and the chuck moving mechanism is varied, or backlash or deformation occurs in the chuck moving mechanism, the correct position of the chuck is fed back to the chuck moving mechanism irrespective of such variation, backlash, or deformation. According to this configuration, it is possible effectively to prevent the approach distance of the chucks from deviating from a desired value.
The position of both chucks maybe detected and, in this case, it is possible further to effectively prevent the approach distance of the chucks from being deviated from a desired value. That is, in this case, a higher accuracy is attained than that attained by detecting the position of only one chuck.
As described above, according to the invention, in the process of forming a light emitting tube portion in an intermediate portion of a cylindrical glass tube blank, the light emitting tube portion is accurately formed. Therefore, the optical quality of an arc tube is enhanced.
The specific configuration of the above-mentioned xe2x80x9cchuck moving mechanismxe2x80x9d is not particularly restricted. However, when the chuck moving mechanism is configured by: a rotary belt having opposed moving portions on which the chucks are respectively fixedly supported; and a chuck pressing mechanism presses one of the chucks that is fixedly supported to the belt, the other chuck is moved in the opposite direction because it is interlocked with the movement of the one chuck. Therefore, the chucks are moved by the same distance in a direction toward, and away from, each other.
When the belt is loosened, the approach movement of each chuck is not equal to that of the other. Therefore, it is important to prevent, as far as possible, the belt from being loosened. When a metal belt is used, elongation of the belt due to changes with age can be made much smaller than that in the case where a timing belt made of rubber or the like is used. Hence, occurrence of loosening of the belt can be effectively suppressed by use of a metal belt.
The gathering operation must be gradually performed in order to prevent the intermediate portion of the cylindrical glass tube blank from being deformed unevenly. Therefore, it is preferable to perform the approaching movement of the chucks in a stepwise manner with a plurality of stations. When such a configuration is employed, detection of an initial position of the chuck may be performed during a period from when the rotary belt mechanism moves to a new station, until a start of the pressing operation by the chuck pressing mechanism in that station. In this case, the approach distance of the chucks can be controlled by detecting the position of the chuck before the chuck is moved by the chuck pressing mechanism, and setting this position as a reference. Even when the approach movement of the chucks is performed in a stepwise manner by a plurality of stations, therefore, it is possible to effectively prevent the approach distance of the chucks from deviating from a desired value.